Filename: 188-bridge-guards.txt
Title: Bridge Guards and other anti-enumeration defenses
Author: Nick Mathewson, Isis Lovecruft
Created: 14 Oct 2011
Modified: 10 Sep 2015
Status: Accepted

1. Overview

   Bridges are useful against censors only so long as the adversary
   cannot easily enumerate their addresses. I propose a design to make
   it harder for an adversary who controls or observes only a few
   nodes to enumerate a large number of bridges.

   Briefly: bridges should choose guard nodes, and use the Tor
   protocol's "Loose source routing" feature to re-route all extend
   requests from clients through an additional layer of guard nodes
   chosen by the bridge.  This way, only a bridge's guard nodes can
   tell that it is a bridge, and the attacker needs to run many more
   nodes in order to enumerate a large number of bridges.

   I also discuss other ways to avoid enumeration, recommending some.

   These ideas are due to a discussion at the 2011 Tor Developers'
   Meeting in Waterloo, Ontario.  Practically none of the ideas here
   are mine; I'm just writing up what I remember.

2. History and Motivation

   Under the current bridge design, an attacker who runs a node can
   identify bridges by seeing which "clients" make a large number of
   connections to it, or which "clients" make connections to it in the
   same way clients do.  This has been a known attack since early
   versions {XXXX check} of the design document; let's try to fix it.

2.1. Related idea: Guard nodes

   The idea of guard nodes isn't new: since 0.1.1, Tor has used guard
   nodes (first designed as "Helper" nodes by Wright et al in {XXXX})
   to make it harder for an adversary who controls a smaller number of
   nodes to eavesdrop on clients.  The rationale was: an adversary who
   controls or observes only one entry and one exit will have a low
   probability of correlating any single circuit, but over time, if
   clients choose a random entry and exit for each circuit, such an
   adversary will eventually see some circuits from each client with a
   probability of 1, thereby building a statistical profile of the
   client's activities.  Therefore, let each client choose its entry
   node only from among a small number of client-selected "guard"
   nodes: the client is still correlated with the same probability as
   before, but now the client has a nonzero chance of remaining
   unprofiled.

2.2. Related idea: Loose source routing

   Since the earliest versions of Onion Routing, the protocol has
   provided "loose source routing".  In strict source routing, the
   source of a message chooses every hop on the message's path.  But
   in loose source routing, the message traverses the selected nodes,
   but may also traverse other nodes as well.  In other words, the
   client selects nodes N_a, N_b, and N_c, but the message may in fact
   traverse any sequence of nodes N_1...N_j, so long as N_1=N_a,
   N_x=N_b, and N_y=N_c, for 1 < x < y.

   Tor has retained this feature, but has not yet made use of it.

3. Design

   Every bridge currently chooses a set of guard nodes for its
   circuits.  Bridges should also re-route client circuits through
   these circuits.

   Specifically, when a bridge receives a request from a client to
   extend a circuit, it should first create a circuit to its guard,
   and then relay that extend cell through the guard.  The bridge
   should add an additional layer of encryption to outgoing cells on
   that circuit corresponding to the encryption that the guard will
   remove, and remove a layer of encryption on incoming cells on that
   circuit corresponding to the encryption that the guard will add.

3.1. Loose-Source Routed Circuit Construction

   Alice, an OP, is using a bridge, Bob, and she has chosen the
   following path through the network:

       Alice -> Bob -> Charlie -> Deidra

   However, Bob has decided to take advantage of the loose-source
   routing circuit characteristic (for example, in order to use a bridge
   guard), and Bob has chosen N additional loose-source routed hop(s),
   through which he will transparently relays cells.

   NOTE: For the purposes of bridge guards, N is always 1.  However, for
   completion's sake, the following details of the circuit construction
   are generalized to include N > 1.  Additionally, the following steps
   should hold for a hop at any position in Alice's circuit that has
   decided to take advantage of the loose-source routing feature, not
   only for bridge ORs.

   From Alice's perspective, her circuit path matches the one diagrammed
   above.  However, the overall path of the circuit is:

       Alice -> Bob -> Guillaume -> Charlie -> Deidra

   From Bob's perspective, the circuit's path is:

       Alice -> Bob -> Guillaume -> Charlie -> UNKNOWN

   Interestingly, because Bob's behaviour towards Guillaume and choices
   of cell types is that of a normal OP, Guillaume's perspective of the
   circuit's path is:

       Bob -> Guillaume -> Charlie -> UNKNOWN

   That is, to Guillaume, Bob appears (for the most part) to be a
   normally connecting client.  (See §4.1 for more detailed analysis.)

3.1.1. Detailed Steps of Loose-Source Routed Circuit Construction

   1. Connection from OP

      Alice has connected to Bob, and she has sent to Bob either a
      CREATE/CREATE_FAST or CREATE2 cell.

   2. Loose-Source Path Selection

      In anticipation of Alice's first RELAY_EARLY cell (which will
      contain an EXTEND cell to Alice's next hop), Bob begins
      constructing a loose-source routed circuit.  To do so, Bob chooses
      N additional hop(s):

      2.a. For the first additional hop, H_1, Bob chooses a suitable
           entry guard node, Guillaume, using the same algorithm as OPs.
           See "§5 Guard nodes" of path-spec.txt for additional
           information on the selection algorithm.

      2.b. Each additional hop, [H_2, ..., H_N], is chosen at random
           from a list of suitable, non-excluded ORs.

   3. Loose-Source Routed Circuit Extension and Cell Types

      Bob now follows the same procedure as OPs use to complete the key
      exchanges with his chosen additional hop(s).

      While undergoing these following substeps, Bob SHOULD continue to
      proceed with Step 4, below, in parallel, as an optimization for
      speeding up circuit construction.

      3.a. Create Cells

           Bob sends the appropriate type of create cell to Guillaume.
           For ORs new enough to support the NTor handshake (nearly all
           of them at this point), Bob sends a CREATE2 cell.  Otherwise,
           for ORs which only support the older TAP handshake, Bob sends
           either a CREATE or CREATE_FAST cell, using the same
           decision-making logic as OPs.

           See §4.1 for more information the distinguishability of
           bridges based upon whether they use CREATE versus
           CREATE_FAST.  Also note that the CREATE2 cell has since
           become ubiquitous after this proposal was originally drafted.
           Thus, because we prefer ORs which use loose-source routing to
           behave (as much as possible) like OPs, we now prefer to use
           CREATE2.

      3.b. Created Cells

           Later, when Bob receives a corresponding CREATED/CREATED_FAST
           or CREATED2 cell from Guillaume, Bob extracts key material
           for the shared forward and reverse keys, KG_f and KG_b,
           respectively.

      3.c. Extend Cells

           When N > 1, for each additional hop, H_i, in [H_2, ..., H_N],
           Bob chooses the appropriate type of extend cell for H_i, and
           sends this extend cell to H_i-1, who transforms it into a
           create cell in order to perform the extension.  To choose
           which type of extend cell to send, Bob uses the same
           algorithm as an OP to determine whether to use EXTEND or
           EXTEND2.  Similar to the CREATE* cells above, for most modern
           ORs, this will very likely mean an EXTEND2 cell.

      3.d. Extended Cells

           When a corresponding EXTENDED/EXTENDED2 cell is received for
           an additional hop, H_i, Bob extracts the shared forward and
           reverse keys, Ki_f and Ki_b, respectively.

   4. Responding to the OP

      Now that the additional hops in Bob's loose-source routed circuit
      are chosen, and construction of the loose-source routed circuit
      has begun, Bob answers Alice's original CREATE/CREATE_FAST or
      CREATE2 cell (from Step 1) by sending the corresponding created
      cell type.

      Alice has now built a circuit through Bob, and the two share the
      negotiated forward and reverse keys, KB_n and KB_p, respectively.

      Note that Bob SHOULD do this step in tandem with the loose-source
      routed circuit construction procedure outlined in Step 3, above.

   5. OP Circuit Extension

      Alice then wants to extend the circuit to node Charlie.  She makes
      a hybrid-encrypted onionskin, encrypted to Charlie's public key,
      containing her chosen g^x value.  She puts this in an extend cell:
      "Extend (Charlie's address) (Charlie's OR Port) (Onionskin)
      (Charlie's ID)".  She encrypts this with KB_n and sends it as a
      RELAY_EARLY cell to Bob.

      Bob's behaviour is now dependent on whether the loose-source
      routed circuit construction steps (as outlined in Step 3, above)
      have already completed.

      5.a. The Loose-Source Routed Circuit Construction is Incomplete

           If Bob has not yet finished the loose-source routed circuit
           construction, then Bob MUST store the first outgoing
           (i.e. exitward) RELAY_EARLY cell received from Alice until
           the loose-source routed circuit construction has been
           completed.

           If any incoming (i.e. toward the OP) RELAY* cell is received
           while the loose-source routed circuit is not fully
           constructed, Bob MUST drop the cell.

           If Bob has already stored Alice's first RELAY_EARLY cell, and
           Alice sends any additional RELAY* cell, then Bob SHOULD mark
           the entire circuit for close with END_CIRC_REASON_TORPROTOCOL.

      5.b. The Loose-Source Routed Circuit Construction is Completed

           Later, when the loose-source routed circuit is fully
           constructed, Bob MUST send any stored cells from Alice
           outward by following the procedure described in Step 6.a.

   6. Relay Cells

      When receiving a RELAY* cell in either direction, Bob MAY keep
      statistics on the number of relay cells encountered, as well as
      the number of relay cells relayed.

      6.a. Outgoing Relay Cells

           Bob decrypts the RELAY* cell with KB_n.  If the cell becomes
           recognized, Bob should now follow the relay command checks
           described in Step 6.c.

           Bob MUST encrypt the relay cell's underlying payload to each
           additional hop in the loose-source routed circuit, in
           reverse: for each additional hop, H_i, in [H_N, ..., H_1],
           Bob encrypts the relay cell payload to Ki_f, the shared
           forward key for the hop H_i.

           Bob MUST update the forward digest, DG_f, of the relay cell,
           regardless of whether or not the cell is recognized.  See
           6.c. for additional information on recognized cells.

           Bob now sends the cell outwards through the additional hops.
           At each hop, H_i, the hop removes a layer of the onionskin by
           decrypting the cell with Ki_f, and then hop H_i forwards the
           cell to the next addition additional hop H_i+1.  When the
           final additional hop, H_N, received the cell, the OP's cell
           command and payload should be processed by H_N in the normal
           manner for an OR.

      6.b. Incoming Relay Cells

           Bob MUST decrypt the relay cell's underlying payload from
           each additional hop in the loose-source routed circuit (in
           forward order, this time): For each additional hop, H_i, in
           [H_1, ..., H_N], Bob decrypts the relay cell payload with
           Ki_b, the shared backward key for the hop H_i.

           If the cell has becomes recognized after all decryptions, Bob
           should now follow the relay command checks described in Step
           6.c.

           Bob MUST update the backward digest, DG_b, of the relay cell,
           regardless of whether or not the cell is recognized.  See
           6.c. for additional information on recognized cells.

           Bob encrypts the cell towards the OP with KB_p, and sends the
           cell inwards.

      6.c. Recognized Cells

           If a relay cell, either incoming or outgoing, becomes
           recognized (i.e. Bob sees that the cell was intended for him)
           after decryption, and there is no stream attached to the
           circuit, then Bob SHOULD mark the circuit for close if the
           relay command contained within the cell is any of the
           following types:

               - RELAY_BEGIN
               - RELAY_CONNECTED
               - RELAY_END
               - RELAY_RESOLVE
               - RELAY_RESOLVED
               - RELAY_BEGIN_DIR

           Apart from the above checks, Bob SHOULD essentially treat
           every cell as "unrecognized" by following the en-/de-cryption
           procedures in Steps 6.a. and 6.b. regardless of whether the
           cell is actually recognized or not.  That is, since this is a
           loose-source routed circuit, Bob SHOULD relay cells not
           intended for him *and* cells intended for him through the
           leaky pipe, no matter what the cell's underlying payload and
           command are.

3.1.2. Example Loose-Source Circuit Construction

   For example, given the following circuit path chosen by Alice:

       Alice -> Bob -> Charlie -> Deidra

   when Alice wishes to extend to node Charlie, and Bob the bridge is
   using only one additional loose-source routed hop, Guillaume, as his
   bridge guard, the following steps are taken:

       - Alice packages the extend into a RELAY_EARLY cell and encrypts
         the RELAY_EARLY cell with KB_f to Bob.

       - Bob receives the RELAY_EARLY cell from Alice, and he follows
         the procedure (outlined in §3.1.1. Step 6.a.) by:

           * Decrypting the cell with KB_f,
           * Encrypting the cell to the forward key, KG_f, which Bob
             shares with his guard node, Guillaume,
           * Updating the cell forward digest, DG_f, and
           * Sending the cell as a RELAY_EARLY cell to Guillaume.

       - When Guillaume receives the cell from Bob, he processes it by:

           * Decrypting the cell with KG_f.  Guillaume now sees that it
             is a RELAY_EARLY cell containing an extend cell "intended"
             for him, containing: "Extend (Charlie's address) (Charlie's
             OR Port) (Onionskin) (Charlie's ID)".
           * Performing the circuit extension to the specified node,
             Charlie, by acting accordingly: creating a connection to
             Charlie if he doesn't have one, ensuring that the ID is as
             expected, and then sending the onionskin in a create cell
             on that connection.  Note that Guillaume is behaving
             exactly as a regular node would upon receiving an Extend
             cell.
           * Now the handshake finishes.  Charlie receives the onionskin
             and sends Guillaume "CREATED g^y,KH".
           * Making an extended cell for Bob which contains
             "E(KG_b, EXTENDED g^y KH)", and
           * Sending the extended cell to Bob.  Note that Charlie and
             Guillaume are both still behaving in a manner identical to
             regular ORs.

       - Bob receives the extended cell from Guillaume, and he follows
         the procedure (outlined in §3.1.1. Step 6.b.) by:

           * Decrypting the cell with KG_b,
           * Encrypting the cell to Alice with KB_b,
           * Updating the cell backward digest, DG_b, and
           * Sending the cell to Alice.

        - Alice receives the cell, and she decrypts it with KB_b, just
          as she would have if Bob had extended to Charlie directly.
          She then processes the extended cell contained within to
          extract shared keys with Charlie.  Note that Alice's behaviour
          is identical to regular OPs.

3.2. Additional Notes on the Construction

   Note that this design does not require that our stream cipher
   operations be commutative, even though they are.

   Note also that this design requires no change in behavior from any
   node other than Bob, and as we can see in the above example in §3.1.2
   for Alice's circuit extension, Alice, Guillaume, and Charlie behave
   identical to a normal OP and normal ORs.

   Finally, observe that even though the circuit N hops longer than it
   would be otherwise, no relay's count of permissible RELAY_EARLY cells
   falls lower than it otherwise would.  This is because the extra hop
   that Bob adds is done with RELAY_EARLY cells, then he continues to
   relay Alice's cells as RELAY_EARLY, until the appropriate maximum
   number of RELAY_EARLY cells is reached.  Afterwards, further
   RELAY_EARLY cells from Alice are repackaged by Bob as normal RELAY
   cells.

4. Alternative designs

4.1. Client-enforced bridge guards

   What if Tor didn't have loose source routing?  We could have
   bridges tell clients what guards to use by advertising those guard
   in their descriptors, and then refusing to extend circuits to any
   other nodes.  This change would require all clients to upgrade in
   order to be able to use the newer bridges, and would quite possibly
   cause a fair amount of pain along the way.

   Fortunately, we don't need to go down this path.  So let's not!

4.2. Separate bridge-guards and client-guards

   In the design above, I specify that bridges should use the same
   guard nodes for extending client circuits as they use for their own
   circuits.  It's not immediately clear whether this is a good idea
   or not.  Having separate sets would seem to make the two kinds of
   circuits more easily distinguishable (even though we already assume
   they are distinguishable).  Having different sets of guards would
   also seem like a way to keep the nodes who guard our own traffic
   from learning that we're a bridge... but another set of nodes will
   learn that anyway, so it's not clear what we'd gain.

   One good reason to keep separate guard lists is to prevent the
   *client* of the bridge from being able to enumerate the guards that
   the bridge uses to protect its own traffic (by extending a circuit
   through the bridge to a node it controls, and finding out where the
   extend request arrives from).

5. Additional bridge enumeration methods and protections

   In addition to the design above, there are more ways to try to
   prevent enumeration.

   Right now, there are multiple ways for the node after a bridge to
   distinguish a circuit extended through the bridge from one
   originating at the bridge.  (This lets the node after the bridge
   tell that a bridge is talking to it.)

5.1. Make it harder to tell clients from bridges

   When using the older TAP circuit handshake protocol, one of the
   giveaways is that the first hop in a circuit is created with
   CREATE_FAST cells, but all subsequent hops are created with CREATE
   cells.

   However, because nearly everything in the network now uses the newer
   NTor circuit handshake protocol, clients send CREATE2 cells to all
   hops, regardless of position.  Therefore, in the above design, it's
   no longer quite so simple to distinguish an OP connecting through
   bridge from an actual OP, since all of the circuits that extend
   through a bridge now reach its guards through CREATE2 cells (whether
   the bridge originated them or not), and only as a fallback (e.g. if
   an additional node in the loose-source routed path does not support
   NTor) will the bridge ever use CREATE/CREATE_FAST.  (Additionally,
   when using the fallback mathod, the behaviour for choosing either
   CREATE or CREATE_FAST is identical to normal OP behaviour.)

   The CREATE/CREATE_FAST distinction is not the only way for a
   bridge's guard to tell bridges from orginary clients, however.
   Most importantly, a busy bridge will open far more circuits than a
   client would.  More subtly, the timing on response from the client
   will be higher and more highly variable that it would be with an
   ordinary client.  I don't think we can make bridges behave wholly
   indistinguishably from clients: that's why we should go with guard
   nodes for bridges.

   [XXX For further research: we should study the methods by which a
   bridge guard can determine that they are acting as a guard for a
   bridge, rather than for a normal OP, and which methods are likely to
   be more accurate or efficient than others. -IL]

5.2. Bridge Reachability Testing

   Currently, a bridge's reachability is tested both by the bridge
   itself (called "self-testing") and by the BridgeAuthority.

5.2.1. Bridge Reachability Self-Testing

   Before a bridge uploads its descriptors to the BridgeAuthority, it
   creates a special type of testing circuit which ends at itself:

       Bob -> Guillaume -> Charlie -> Bob

   Thus, going to all this trouble to later use loose-source routing in
   order to relay Alice's traffic through Guillaume (rather than
   connecting directly to Charlie, as Alice intended) is diminished by
   the fact that Charlie can still passively enumerate bridges by
   waiting to be asked to connect to a node which is not contained
   within the consensus.

   We could get around this option by disabling self-testing for bridges
   entirely, by automatically setting "AssumeReachable 1" for all bridge
   relays… although I am not sure if this is wise.

   Our best idea thus far, for bridge reachability self-testing, is to create
   a circuit like so:

       Bridge → Guard → Middle → OtherMiddle → Guard → Bridge

   While, clearly, that circuit is just a little bit insane, it must be that
   way because we cannot simply do:

       Bridge → Guard → Middle → Guard → Bridge

   because the Middle would refuse to extend back to the previous node
   (all ORs follow this rule).  Similarly, it would be inane to do:

       Bridge → Guard → Middle → OtherMiddle → Bridge

   because, obviously, that merely shifts the problem to OtherMiddle and
   accomplishes nothing.  [XXX Is there something smarter we could do? —IL]

5.2.2. Bridge Reachability Testing by the BridgeAuthority

   After receiving Bob's descriptors, the BridgeAuthority attempts to
   connect to Bob's ORPort by making a direct TLS connection to the
   bridge's advertised ORPort.

   Should we change this behaviour?  One the one hand, at least this
   does not enable any random OR in the entire network to enumerate
   bridges.  On the other hand, any adversary who can observe packets
   from the BridgeAuthority is capable of enumeration.

6. Other considerations

   What fraction of our traffic is bridge traffic?  Will this alter
   our circuit selection weights?
